1. Field of the Invention
This invention relates generally to the field of amplifiers and more particularly to switchable gain amplifiers.
2. Related Art
One of the most readily appreciated benefits of wireless communications is the mobility afforded the user of wireless communications. Mobile handsets having wireless telephone technology may be used in a variety of environments, for example, indoors, outside, or while travelling in an automobile or other vehicle.
The mobile use of a wireless communication devices, however, may result in large variations in signal strength of the radio frequency (RF) signals received by the wireless communication devices. Depending upon where the mobile handset is in relation to a basestation, and what obstacles may be in the transmission path, the received RF signal may be anywhere within a very wide range of signal strengths at the antenna. When the signal is relatively weak, more amplification in the radio is required. When the signal is strong, less amplification is required. Accordingly, wireless communications devices have been provided with low noise amplifiers (LNAs) for the RF signal that have a gain that can be adjusted in response to the strength of the received RF signal.
Some standards for wireless communications are more sensitive to variations in signal strength than others. For example, Code Division Multiple Access (CDMA) technology is especially sensitive to variations in RF signal strength. Thus, for a CDMA signal, high linearity amplification is highly desirable because it does not have a constant envelope. RF signal strength variations, however, may affect other formats of wireless communication to one degree or another, and the present invention is not necessarily limited to any particular communications format.
Previous solutions to providing a low noise amplifier (LNA) with adjustable gain include discrete attenuators or by simply switching off the LNA. For example, one known solution uses a discrete attenuator selectively switched in series with the input of the LNA. When the received RF signal is strong, the attenuator is switched in, and the incoming signal is attenuated before going to the LNA. When the received signal is relatively weak, the attenuator is switched out. A discrete attenuator, however, has the disadvantages of additional material costs, additional area required on the circuit board of the wireless device, and additional load on the input of the LNA even when the attenuator is switched out, thereby degrading noise performance.
Simply switching off the LNA also has a drawback. The RF signal output of the LNA is typically demodulated into baseband level signals for further processing. One advantageous use of a driver circuit to the downconverter is to use the driver circuit as a biasing current sink for the LNA. Switching off or bypassing the LNA when a strong signal is present would not allow the dual use of the driver for the downconverter as a current sink for the LNA.